Lot's of re-sizing going on!

I've just started on a campaign to resize many of my AVI's the way I like them. For me that means (In Windows Media Player) a letterbox "look" to the video, even if it isn't a letterbox, with about one half inch of black bars on the top and the bottom.

Some of my AVI's reach the top of the screen ok, but have black bars on the right and left sides. I have found them harder to turn into a "letterbox" look. Why is that?

Before I ask any more questions, I want to double-check and make sure I'm using AVIdemux correctly. I open a file in the program and select, under video "MPEG-4 ASP (Xvid4), then I choose "Filter" and "Re-size", set my desired size and then close that screen, save it with a .avi extension and then let it do it's thing.

I have noticed that some AVI', like some with real small frame settings of like 320 x 240, cannot be re-sized, even if I set my target at say, 640 x 360. Why is that?

Also I've been getting into the habit of going into the AVI files "Properties" and "Details" and checking the frame height and width. One thing I don't understand is, why do some files with small frame sizes, like 570 x 300 look just about as full-screen in WMP as a file with a larger frame size? Is that just WMP doing it's job of expanding it?

Another thing I noticed that even after I use AVIdemux and expand a files size, when I go into "Properties" and "Details" the file still lists the original frame size. Why is that?

One more thing, then I will stop....When your re-sizing in AVIDemux, does your finished project have any loss of quality from the original?

Thanks in advance and sorry for so many questions. Just trying to increase my level of expertise in this area!